were not ill more often than those who received the low-titer vaccine. This was also true for girls.

Ward noted that all children in both studies had measles antibodies by age 5, even though 10 to 20 percent of these children had not developed antibodies to measles shortly after they received the measles vaccine when they were infants. These initially seronegative children were likely to have been infected with natural measles, which was circulating in Sudan and Haiti, between the time that these children were vaccinated and the time that they were followed up. This suggests that any effects observed in the study may be due, in part, to natural infection with measles and not to the measles vaccine received.

Chen noted that one study indicates that there may be delayed mortality after a natural measles infection (Hull et al., 1983). This delayed mortality might be mimicked by the measles vaccine with a high antigen titer. The delayed mortality associated with measles infection might occur more often in girls, Ward said, because studies suggest that measles causes more deaths in girls than in boys (Garenne et al., 1991). Such excess delayed mortality is not seen in all outbreaks (Chen et al., 1994).

Both multiple sclerosis (MS) and Guillain-Barré syndrome (GBS) are thought to be associated with certain vaccines (IOM, 1994) but the causal mechanisms of these adverse effects are not known and somewhat speculative. On the presumption that there is an association, some possible mechanisms worth exploring were discussed at the workshop.

The hallmark of MS, which affects between 250,000 and 350,000 people in the United States (Kurtzke, 1997), is patchy destruction of the fatty sheath, called myelin, that envelops and insulates the nerves in the brain and spinal column. This destruction of myelin disrupts the passage of messages along these nerves, causing a number of symptoms that include weakness, tingling, numbness, loss of coordination and balance, dizziness, muscle spasms, slurred speech, and blurred or double vision. The cause of MS is unknown, although it is thought to be an autoimmune disorder that afflicts a subgroup of people who are genetically susceptible. According to McFarland, studies have failed to demonstrate that MS is caused by a molecular mimicry mechanism.

The symptoms of MS tend to wax and wane over time, and magnetic resonance imaging (MRI) studies suggest that patients have central nervous

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This section is based on information presented by Henry McFarland and Dale Lawrence.